I've been using GTDTiddlyWiki as my private offline wiki for over a year now and I was pretty happy with it for a long time.
At the time when I picked GTDTiddlyWiki as my private wiki I didn't realize how much and how often I would use it. It's about that time when my needs have outgrown what GTDTiddlyWiki has to offer, so I'm looking around for something with a few more features and ongoing development.
I like the idea of a client-side wiki as a private wiki, and TiddlyWiki and all its flavors seems to dominate this area. I was considering switching to TiddlyWiki, but I found a very interesting feature called TagglyTagging of MonkeyPirateTiddlyWiki (yet another TiddlyWiki flavor). I though that content tagging couldn't get any better, but it seems that I was wrong. :)
I'll give MonkeyPirateTiddlyWiki a shot!
BTW tiddlyspot is hosting TiddlyWiki, MonkeyPirateTiddlyWiki and a few other TiddlyWiki flavors. By using their service you can easily upload your wiki to their servers and make it available to the public (or password protect it). Pretty neat, but after being among security geeks at Sun for some time I'm hesitant to use this cool feature. :-/
Am I getting paranoid enough Martin? ;-)